The most widely used precision AC measurment makes use of the AC/DC transfer method. The principle is based on measuring some reliable and convertible characteristic such as the RMS of an AC signal under known and controlled conditions and then producing a measurement of equal magnitude by application of pure DC voltage and precisely measuring the DC. The device used for converting an AC signal to a DC signal under known and controlled conditions is a differential multijunction thermal converter (MJTC). The article "Differential multijunction thermal converter" by A. J. Wilkins et al, Proceedings of IEE Vol. 112, No. 4, p. 794, April 1965 provides a detailed description of the thermal converter. U.S. Pat. No. 3,842,349 Oct. 15, 1974, (Cox et al) describes an AC/DC RMS comparator. Briefly speaking, in the instrument of Cox et al, an unknown AC voltage and a reference DC voltage are alternately applied to one side of a MJTC by using electronic switches. A feedback circuit with an integrator applies a nulling voltage to the other side of the converter. The output of the integrator is connected to one Sample/Hold circuit when the unknown AC voltage is applied to the converter and to another Sample/Hold circuit when the reference DC voltage is applied to the converter. If the unknown AC voltage and the reference DC voltage have the same RMS value, Sample/Hold circuits are in balance. The value of the unknown AC voltage is obtained by measuring the voltage of the DC reference.
Although this instrument is said to be automatic, it requires 4 manual operations to achieve a high accuracy measurement. These operations are reversal of the DC reference, i.e., DC+ and DC-, and interchange of the input channels, i.e., Normal and Reverse. Thus to determine the value of the unknown AC voltage 4 manually controlled measurements are necessary.
U.S. Pat. No. 3,845,388 Oct. 29, 1974 (Ley et al) discloses an RMS converter wherein a voltage-to-frequency converter is used in a feedback circuit which operates in the digital fashion. However this instrument is called single-ended in that only an unknown input AC signal is put through a MJTC and a converter output is compared with a DC reference. As mentioned above, in the device of Cox et al, an unknown AC signal and a DC reference are alternatively applied to the same heater of the MJTC and the difference in the converter outputs is used to determine the RMS value of the AC signal.
An automated AC-DC difference calibration system is described in the article "A dual-channel automated comparator for AC-DC difference measurements" by Williams et al in IEEE Trans. on Instrumentation and Measurement, Vol. IM-34 No. 2, June 1985 pp. 290-294. The comparator consists essentially of two practically identical channels, each containing a thermal voltage converter. All AC and DC signals are simultaneously applied to the channels and the difference between them is measured and displayed. Though fast in operation, the system requires extensive instrumentation.